Robert Frost's poem "Fireflies in the Garden" is a brief but eminently clear commentary on the limits of symbolism. Frost uses fireflies as symbols which "emulate...real stars...(in) the upper skies". He notes that "though they were never equal stars in size, and they were never really stars at heart", they do at times actually "achieve...a very star-like start". The only problem is, unlike the real thing, fireflies "can't sustain the part".
Through this metaphor, Frost is saying that although symbols are quite effective at times, almost duplicating that which they stand for, their impact is limited, because they attain closeness to their subject for only a fleeting moment. Symbols cannot sustain their effectiveness for any length of time; their lustre quickly pales in the face of the real thing.
In essence, symbols are ephemeral, and their transience limits them. They may be beautiful and convincing, like the fireflies who "Achieve at times a very star-like start," but once the poem is complete, the meanings of the symbol fade, like the light of a firefly. The star can just keep on shining, it seems for eternity, but the firefly cannot; the lifespan of its light is so incredibly short compared to how long the star can shine. This poem, in fact, is relatively short, and its clipped length seems to actually mimic the truncated length of a symbol's figurative power compared to the literal stability of the thing it symbolizes. Frost's poem seems to say that the symbol may aim high, but, at best, it can only "emulat[e]" and "never equal" the permanence of whatever it represents.